(1) Field of the Invention
The present invention relates generally to a method for coating urea, and in particular to a method for coating urea pellets or granules with phosphate to reduce nitrogen loss while providing a source of phosphorous. The invention also relates to the resultant products.
(2) Description of the Prior Art
Nitrogen and phosphorus are two of the primary nutrients for plant growth (Marshner, H. The Mineral Nutrition of Higher Plants. Second edition. Academic Press San Diego, Calif. 1995). Nitrogen is required by all plants to produce the proteins, DNA and RNA for growth and development. Phosphorus is required for energy metabolism and to build the backbone of the DNA and RNA molecules. Many approaches have been developed which can supply these nutrients to plants.
Urea is a widely used, non-burning source of nitrogen for plants and is generally sold in pelletized or granular form. When broadcast on the soil, the urea dissolves by absorbing water from sources such as irrigation, rain, or moisture from the air, dispersing the urea into the soil so that it is available to the plant.
Unfortunately, urea is subject to attack by urease enzymes from soil bacteria, which can lead to significant loss of the nitrogen available in urea. This attack by urease leads to the release of ammonia gas to the air by the reaction known as volatilization of nitrogen according to the following equation: EQU NH.sub.2 CONH.sub.2 +H.sub.2 O.fwdarw.2NH.sub.3 +CO.sub.2 (1)
Nitrogen losses due to volatilization can lead to as much as an 80% reduction in the available nitrogen applied as urea (Terman, G. L. Volatilization Losses of Nitrogen as Ammonia from Surface Applied Fertilizers, Organic Amendments, and Crop Residues. Advances in Agronomy 31: 189-223, 1979.) The loss of nitrogen by volatilization can be an important economic loss to the farmer or forester, since replacement of the lost nitrogen requires additional applications of nitrogen fertilizers, increasing crop production costs, and the potential for nitrogen run-off that can pollute surface water supplies.
Phosphorous can be supplied to the plant in a number of ways. Ammonium phosphate salts can be prepared by reacting ammonia with phosphoric acid by technology that is well established. Both solid and liquid products are made from ammonium salts, and are widely used to supply both nitrogen and phosphorus. Soluble calcium phosphate salts such as calcium dihydrogen phosphate (Ca(H.sub.2 PO.sub.4).sub.2, monocalcium phosphate) can be used to supply phosphorus. This salt which provides both phosphorus and calcium to the plant must be produced from bone meal or phosphate mineral deposits.
In the fertilizer industry, the total phosphorus content of a fertilizer product is typically expressed in terms of percent P.sub.2 O.sub.5. The term "available P.sub.2 O.sub.5 " refers to that phosphorus which can be extracted with citric acid from a phosphorus source. The term "insoluble P.sub.2 O.sub.5 " is the difference between total P.sub.2 O.sub.5 and available P.sub.2 O.sub.5. The term "water-soluble P.sub.2 O.sub.5 " refers to that phosphorus which is extractable with water from a phosphorus source. To remain consistent with industry practice, phosphorus content is expressed herein as percent P.sub.2 O.sub.5 and will refer to total phosphorous content unless designated otherwise.
Phosphorus mineral deposits exist in nature as highly insoluble fluoroapatites (CaF.sub.2. 3 Ca.sub.3 (PO.sub.4).sub.2). Each phosphate deposit in the world has a slightly different composition that is given as the total P.sub.2 O.sub.5 content of the mineral obtained from the deposit. Major phosphorus deposits are found in Morocco and Jordan, and in the U.S. in Florida, North Carolina, and Utah.
Monocalcium phosphate can be prepared by reacting phosphoric acid with the fluoroapatite mineral according to Equation 2 (Austin, G. T. editor. Shreve's Chemical Process Industries fifth edition. McGraw Hill Book Company. New York, N.Y. 1984).
The use of the reaction shown in Equation 2 to produce triple superphosphate (fertilizer grade monocalcium phosphate, 46% available P.sub.2 O.sub.5) is well known. EQU CaF.sub.2. Ca.sub.3 (PO.sub.4).sub.2 +14H.sub.3 PO.sub.4 .fwdarw.10Ca(H.sub.2 PO.sub.4).sub.2 +2HF
Other acids can be reacted with a fluoroapatite mineral. Sulfuric acid will form calcium sulfate and phosphoric acid when reacted with a fluoroapatite. Nitric acid will form calcium nitrate and phosphoric acid. Organic impurities in fluoroapatite minerals make sulfuric acid a better choice than oxidizing acids such as nitric acid. Organic acids, such as citric acid used in the test for available phosphorus, can also be used.
Coating urea pellets with an acid or acidic substance is known in the prior art. U.S. Pat. No. 4,073,633 teaches that it is possible to control the pH in the area so that the urea dissolves to slow the volatilization. This is accomplished by contacting urea with an acid or an acid salt. Many acids including phosphoric acid, sulfuric and nitric acid will work according to the '633 patent. Metal salts, which produce a solution with a pH of 5 or less such as ferric nitrate, ferric chloride, calcium dihydrogen phosphate can be used according to the '633 patent. The calcium dihydrogen phosphate disclosed in the '633 patent is the purified salt which is first dissolved in water. Kiseleglur or clay may be added to aid in the adhesion of the acid or aqueous solution and to ensure that a free flowing particle is formed.
Australian Patent No. AU 9645576 discloses that metal salts may be coated onto the surface of urea by first wetting the surface with water then mixing with tumbling action the dry salts with the wet urea. The following patents also describe methods whereby granular fertilizers may be coated: U.S. Pat. No. 3,423,199, U.S. Pat. No. 3,520,651, U.S. Pat. No. 3,560,192, U.S. Pat. No. 3,961,932 and U.S. Pat. No. 5,152,821.
The prior art teaches a number of methods whereby the decomposition of urea by soil enzymes can be prevented. One method is to provide a controlled release formulation, which can be achieved by reacting urea with and aldehyde (U.S. Pat. No. 3,322,528), or forming a polymeric structure that results in controlled release (U.S. Pat. No. 4,752,317). Protein degraded pre-vulcanized rubber, also, has been used to form urea fertilizers with slow release characteristics (U.S. Pat. No. 4,549,897).
Coatings that employ urease inhibitors such as phosphoric triamide compounds are very effective at inhibiting urease (U.S. Pat. No. 4,530,714). Improvements of this idea are presented in (WO 97/22568). A potential draw back to this approach is the cost of the inhibitor.
There is still a need, however, for an inexpensive and effective method of coating urea to reduce the loss of nitrogen through volatilization, while also providing a source of phosphorous, as well as a need for the resultant coated urea products.